Security screening method and apparatus

ABSTRACT

A method and apparatus to screen individuals specifically for paramagnetic or ferromagnetic objects they may be carrying or wearing, before they enter a security area. The device comprises either a screening portal or a compact, hand-held magnetic gradiometer and its electronics. The device places all of the sensor arrays in close proximity to all parts of a subject&#39;s body, for screening purposes.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a continuation application of co-pending U.S. App. Ser.No. 10/681,033, filed Oct. 7, 2003, for “Magnetic Resonance ImagingScreening Method and Apparatus”. This application relies upon U.S.Provisional Pat. App. No. 60/440,697, filed Jan. 17, 2003, for “Methodand Apparatus to Use Magnetic Entryway Detectors for Pre-MRI Screening”,and U.S. Provisional Pat. App. No. 60/489,250, filed Jul. 22, 2003, for“Ferromagnetic Wand Method and Apparatus for Magnetic Resonance ImagingScreening”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention is in the field of methods and apparatusused to prevent the presence of paramagnetic or ferromagnetic objectsnear a magnetic resonance imaging (MRI) system.

[0005] 2. Background Art

[0006] Paramagnetic and ferromagnetic objects are highly unsafe near MRIsystems, because the strong magnetic gradients caused by MRI magnetsexert a strong force on such objects, potentially turning them intodangerous missiles. Several accidents, some fatal, are known to haveoccurred as the result of someone inadvertently carrying such an objectinto the MRI room. Current MRI safety practices rely on signage andtraining to prevent people from taking such objects into the MRIchamber. There is currently no known technical means in use to preventthe accidental transportation of such objects into the MRI chamber, oreven to warn of such an occurrence.

[0007] Use of conventional metal detectors, whether portals or wands,would not be efficient for this purpose, because they do not distinguishbetween magnetic and non-magnetic objects, and only magnetic objects aredangerous. Conventional systems generate an audio-band oscillating orpulsed magnetic field with which they illuminate the subject. Thetime-varying field induces electrical eddy currents in metallic objects.It is these eddy currents which are detected by the system, to revealthe presence of the metallic objects. There is no discrimination betweenferromagnetic objects, which are dangerous near an MRI system, andnon-magnetic objects, which are not. As a result, conventional systemswould generate far too many false alarms to be usable in thisapplication. The invention described herein solves the problem bydetecting only paramagnetic and ferromagnetic objects, which are exactlythose that must be excluded from the MRI room.

BRIEF SUMMARY OF THE INVENTION

[0008] The present invention provides an apparatus and a method forscanning a patient or attendant for the presence of an object which iseither permanently magnetic or susceptible to being magnetized by anexternal field. The sensors in this scanning apparatus can be mounted oneither a wand type frame, or a portal type frame. Either embodimentpositions the entire sensor array in proximity to every portion of apatient or other individual. The wand embodiment of the scanner can bepassed in proximity to every portion of the subject's body. The portalembodiment of the scanner arranges the sensors in a horizontalalignment, making the sensor array suitable for positioning every sensorin proximity to the body of a recumbent patient, as the patient passesthrough the portal.

[0009] The sensors can detect the magnetic field of the object, whetherthe object is a permanent magnet or merely susceptible to magnetization.Where an external field induces a magnetic field in the object, theexternal field may be the Earth's magnetic field, or it may be generatedby another source, such as a nearby MRI apparatus or a dedicated sourcesuch as one mounted on the frame of the apparatus.

[0010] The novel features of this invention, as well as the inventionitself, will be best understood from the attached drawings, taken alongwith the following description, in which similar reference charactersrefer to similar parts, and in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0011]FIG. 1 is a schematic showing the horizontal arrangement of sensorarrays in a first portal type embodiment;

[0012]FIG. 2 is a schematic of a second portal embodiment;

[0013]FIG. 3 is a schematic of a third portal embodiment;

[0014]FIG. 4 is a schematic of a first wand embodiment;

[0015]FIG. 5 is a schematic of a second wand embodiment;

[0016]FIG. 6 is a schematic of a third wand embodiment; and

[0017]FIGS. 7 through 10 are schematics of several embodiments of thearrangement of the source fields and sensors.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention, which applies to both permanently magneticobjects called “hard” ferromagnets and non-permanent magneticallysusceptible objects called “soft” ferromagnets, can use magnetometerswith good sensitivity at frequencies all the way, or nearly, to DC,i.e., zero frequency. This allows several modes of use:

[0019] (1) As a completely passive system, the present invention detectsferromagnetic objects using their permanent magnetization, in the caseof “hard” ferromagnets, or the magnetization induced by the Earth'smagnetic field, in the case of “soft” ferromagnets.

[0020] (2) As a DC magnetic susceptometer, the present invention appliesa static DC magnetic field, allowing control and usually enhancement ofthe magnetization of soft ferromagnets, thus enhancing theirdetectability.

[0021] (3) As an AC magnetic susceptometer, the present inventionapplies an oscillating AC magnetic field, but at very low frequenciescompared to conventional detectors, allowing enhancement of theirmagnetization. The purpose of AC illumination is to move the signal fromDC to a region of lower noise at finite frequency. The AC frequency ischosen to avoid inducing the electrical eddy currents detected by othersystems, to suppress the response from non-ferromagnetic metal objects,and thus maintaining the discrimination capability.

[0022] The present invention importantly arranges an array of sensors insuch a way that the entire sensor array can be placed in proximity toall portions of the body of a subject, such as a patient or anattendant. In particular, the sensor arrays are arranged so as to besusceptible to placement in proximity to all portions of the body of apatient lying recumbent, as on a stretcher or gurney. This object isaccomplished by either of two major embodiments of the invention: aportal structure, and a hand held wand. The portal structure is designedto have one or more horizontally arranged sensor arrays, suitable foralignment of the entire sensor array with a recumbent patient. Thisdiffers from a portal arrangement in which the sensor arrays arearranged vertically, placing only a few of the sensors in proximity to arecumbent patient. The wand is susceptible to movement over the body ofthe subject in order to place the entire sensor array in proximity toall portions of the subject's body.

[0023] A passive magnetic embodiment of the portal used in oneembodiment of the present invention can be similar in some respects tothe SecureScan 2000™ weapons detection portal which is manufactured byQuantum Magnetics, Inc., and marketed by Milestone Technology, Inc., orthe i-Portal™ weapons detection portal which is marketed by QuantumMagnetics, Inc. In important respects, however, the portal would bemodified to be suitable for use in the present invention, namely, tomake it suitable for use with a recumbent subject lying on a gurney orstretcher, rather than walking upright. In the known configuration,patients on gurneys would be too distant from too many of the sensorsfor adequate detection.

[0024] The portal includes two panels of sensors on the sides of theentryway. An array of magnetometers inside each panel enables detection,characterization, and localization of ferromagnetic objects from thesoles of the feet to the top of the head. The magnetometer array cantake a variety of configurations, and it can use a variety of sensortechnologies. For example, a set of 16 single-axis magnetic gradiometerscan be arranged with 8 in each panel. Other configurations can includearrays of multi-axis gradiometers, or combinations of single-axis andmulti-axis gradiometers. One or more magnetic tensor gradiometers mayalso be used. A magnetoresistive magnetometer, or any other sensorcapable of sensing magnetic field changes at or near zero frequency, canbe used.

[0025] As shown in FIG. 1, in order to scan a patient on a gurney, theportal sensor configuration 10 of the present invention must be arrangedto bring all of the sensors closer to the patient and to effectivelyscan a patient in the recumbent position. Rather than being arrangedvertically as in the aforementioned known portals, the two sensor panels12, 14 can be arranged horizontally, parallel to the path of the gurneyand on either side, as shown in FIG. 1. This places the sensors in asimilar relation to the patient as they would have, in the verticalarrangement, to an ambulatory patient. Also, a single “snapshot” of datacovers the entire gurney and patient, as in the ambulatory case. Thesensor panels 12, 14 can be permanently arranged horizontally, or theycan pivot to this configuration.

[0026] Alternatively, in addition to the vertically arranged sensorpanels as in the aforementioned known portals, the portal can have a“dutch door” with an additional, horizontal, sensor panel 16 in theupper half of the door, just high enough to clear a patient on a gurney,as shown in FIG. 2. As the patient is wheeled under the upper door, thepatient would pass in close proximity to the horizontal sensor panel 16,allowing all of its sensors to scan the patient from head to foot, orvice versa. This gives the best detection and resolution of objects,since more sensors are placed closer to the patient. Then, the attendantwould push the dutch door open and walk through the portal, beingscanned by the vertically arranged sensor panels. The “dutch door” array16 can be spring loaded, so that it moves out of the way for anambulatory subject. A microswitch indicator can tell the softwarewhether the door is engaged, for a recumbent patient, or disengaged, foran ambulatory subject. As a variation of this embodiment, a portal withvertically arranged sensor panels can be situated next to a portal witha horizontally arranged sensor panel, as shown in FIG. 3.

[0027] As an alternative to the passive magnetic portal, an AC or DCmagnetizing field can be provided by one or more source coils, a DCfield can be provided by a permanent magnet array, or a DC field can beprovided in the form of the fringing field of a nearby MRI magnet. Inany case, a computer is provided to interrogate the sensors and tointerpret the magnetic signals, to detect, characterize, and locateferromagnetic objects. Characterization of the object provides the sizeand orientation of its magnetic moment, which can be related to thephysical size of the object, and to the magnitude of the attractivemagnetic force. The analysis software can use various known algorithms,or a neural network can be used. The information gained can be relatedto a photographic image of the subject, for the purpose of locating theferromagnetic object on the subject. A light display can be used toindicate the approximate location of the detected object. Systemdiagnosis, monitoring, and signal interpretation can be done via theInternet, if desired.

[0028] As an alternative to the portal type screening apparatus, ahand-held device can be used to screen individuals specifically forstrongly paramagnetic or ferromagnetic objects they may be carrying orwearing, before they enter the high-field region of an MRI suite. Insome instances, the lack of floor space precludes a fixed installationsuch as the portal disclosed above. In these cases, a hand wand may bethe preferred embodiment.

[0029] The hand-held device, or wand, comprises a compact magneticgradiometer and its electronics. The gradiometer can measure either asingle gradient component, multiple components, or the complete gradienttensor. The gradiometer comprises one or more pairs of magnetic sensorsand reads out the difference signal between members of each pair.Background fields have small gradients, so the difference signalresulting from these is small. Close to a paramagnetic or ferromagneticbody, however, field gradients are strong; they vary as 1/r⁴ with thedistance r from the sensor to the magnetic body. A strong anomaly issensed whenever the wand is passed close by such an object of interest.The wand does not detect nonmagnetic metals. Its electronics read thesignals out and process them. The output can be in the form of a simplealarm when the signal exceeds a threshold. More robust processingalgorithms can incorporate adaptive background cancellation to furthersuppress background gradient interference, and target-objectlocalization in the case of full tensor gradiometer implementations.

[0030] To increase the signal from the target object, it can bedesirable to make the measurement in a stronger ambient field than theearth's magnetic field, which is about 0.5 Gauss. The fringing fieldfrom a magnetic resonance imaging (MRI) magnet can provide such anenhanced field, with strengths in excess of 10 Gauss.

[0031] A further embodiment combines the magnetic wand with a wire coilthat can be used, by means of driving electric current through it, togenerate a controlled source field. The coil can be configured tosuppress its common-mode signal on the gradiometer sensors but provide amagnetizing field around the wand. This field, by magnetizingparamagnetic or ferromagnetic objects, increases their signal relativeto the background. The field can be static (DC) or time-varying (AC).The benefit of an AC field is that the system can work at a non-zerofrequency, further suppressing background interference. The frequency ischosen to be low enough, however, not to excite a response fromconductive but nonmagnetic objects.

[0032] This device consists of a rigid, non-metallic, non-magneticstructure that supports one or more pairs of magnetometers. Each pairconsists of sensors aligned to measure the same component of themagnetic field. Each pair's two outputs are differenced to create thegradient signal. Sensor electronics operate the sensors and perform thedifferencing. They also operate signal processing algorithms to suppressbackground interference and to alarm in the proximity of paramagnetic orferromagnetic objects.

[0033] In embodiments involving an active magnetic source, the wand alsohas one or more coils of wire and electronics to drive controlledcurrents in the coils, to act as a magnetizing source field. The coilsare designed to produce a zero differential signal on the gradiometers,in the absence of nearby magnetic objects.

[0034] In a further embodiment, an applied DC magnetic field can becreated by means of one or more permanent magnets mounted in the wand.The magnets are mounted such that their primary magnetic field isoriented orthogonally to the sensitive axis of the magnetometers in thewand. In this way, the sensors are not saturated by the applied DCfield, but remain sensitive to enhanced magnetization of a ferromagneticobject by that field. Use of permanent magnets to generate the field hasan advantage over using a coil, namely, the permanent magnet draws nopower. However, a potential disadvantage is that the magnetic fieldcannot be turned off, so the wand must be stored carefully when not inuse.

[0035] The use of AC fields enables the use of induction coil sensors,in addition to or instead of magnetometers, like magnetoresistive,fluxgate, and other types. Induction coil sensors are impossible to usein the DC embodiment because the induction coil has zero sensitivity atzero frequency. Using induction coil sensors typically reduces the costof the product without sacrificing sensitivity in the AC system. Usinginduction coil sensors confers a particular advantage, in that itrenders the wand insensitive to interference from noise induced by thewand's motion in the Earth's field. This is a major potential source ofinterference in the case of the DC applied field.

[0036] An AC system could make use of two different excitationdirections—operating at two different frequencies, to avoidcrosstalk—which can improve detection of long, narrow objects, which areprecisely the shape that is most dangerous in this situation.

[0037] The wand can be extended into a two-dimensional array of sensorsto enable reliable scanning without as much moving of the wand back andforth. Too large an array becomes unwieldy and expensive; the optimumarray size depends upon the balance between cost, reliability, and userskill found in any given application.

[0038]FIGS. 4 and 5 illustrate the principles of the wand embodiments 20of the invention, utilizing an AC source. An excitation coil 22, bymeans of a sinusoidal (AC) current driven in it, generates analternating magnetic field that excites a combination of magnetizationcurrent and electrical eddy current in any conductive and/orferromagnetic and/or magnetically permeable body nearby. The excitationfrequency is chosen to be low enough so that the magnetization (or,equivalently, magnetic susceptibility) response of objects to bedetected exceeds their eddy current response. The choice of frequencyremains to be determined, but it is expected to be several tens of hertz(Hz), or at least substantially less than 1 kHz.

[0039] The excitation current can be driven by any number of standarddrive circuits, including either direct drive (controlled voltage sourcein series with the coil) or a resonant drive (voltage source coupled tothe coil via a series capacitance whose value is chosen such that, incombination with the coil's self-inductance, the current is a maximum ata desired resonant frequency given by ½π(LC)^(1/2)).

[0040] In both FIGS. 4 and 5, the receiver or sensor coil is, in fact,made of two coils 24A and 24B, wound in opposite senses and connected inseries. They form what is well-known as a gradiometer; a uniformmagnetic flux threading both coils produces zero response. Coils 24A and24B are distributed symmetrically about the excitation coil 22 suchthat, in the absence of any target object (which is conductive, magneticor magnetically permeable) nearby, each senses an identical flux fromthe excitation which thus cancels out. A handle 28 can contain theelectronics and a battery.

[0041] Although the intent is to make the two coils 24A and 24Bperfectly identical, and to place them in identically symmetriclocations, in practice one falls short of the ideal. As a result, anyactual embodiment will display a nonzero response to the excitation,even in the absence of a target; this residual common-mode signal isreferred to as an “imbalance” signal. Standard electrical circuits canzero out the imbalance signal by adding an appropriately scaled fractionof the reference voltage V_(ref) (a voltage proportional to theexcitation current, obtained by measuring across a series monitorresistor) to the output voltage V_(out).

[0042] When a target object is near to either coil 24A or 24B, it spoilsthe symmetry and thus induces a finite signal. This signal oscillates atthe same frequency as the excitation. Standard demodulation orphase-sensitive detection circuits, using V_(ref) as the phasereference, measure the magnitude of V_(out) in phase with V_(ref) and inquadrature (90 degrees out of phase) with V_(ref). At an appropriatelychosen low frequency, the response will be dominated by thesusceptibility response, which appears predominantly in the quadratureoutput, as opposed to the eddy current response, which appearspredominantly in the in-phase component.

[0043] In principle, the coils 24A and 24B could be replaced by twomagnetometer sensors (fluxgate, magnetoresistive, magnetoimpedance,etc.). Coils respond to the time derivative of the magnetic field, whilemagnetometers respond to the field itself; the coil's output voltage isshifted by 90 degrees with respect to a magnetometer's. If magnetometersare used instead of coils, then the susceptibility response would showup in the in-phase component and the eddy current response (at lowfrequency) in the quadrature component.

[0044] If the operating frequency is chosen much too high, bothsusceptibility and eddy-current responses appear in the in-phasecomponent (using magnetometers) or quadrature component (using coils),but with opposite sign, making it impossible to distinguish between thetwo. At intermediate frequencies, the eddy current phase is intermediatebetween the two components, complicating the distinction. Therefore, itis important to choose the excitation frequency to be low enough, andpreferably less than about 1000 Hz.

[0045] The substrate or coil form 26 must be nonconductive,nonferromagnetic and, with one possible exception, magneticallyimpermeable (μ=μ₀, where μ₀ is the permeability of free space). Theexception is that a magnetically permeable core inside the sense coils24A, 24B (practical only in the cylindrical geometry of FIG. 4) canincrease the sensitivity of the system.

[0046] Using a resonant drive circuit for the excitation coil 22 maysignificantly reduce the electrical power needed to create theexcitation. Thus, this embodiment may be preferred for abattery-operated, hand-held wand. The other circuits, includingdemodulation, threshold, discrimination, and alarm/alert, requirenegligible power, so the system power is dominated by the excitationrequirement.

[0047] As shown in FIG. 6, the DC embodiment of the wand 30 can have asensor board with 2 sensors 34, which can be placed at each end of anepoxy fiberglass paddle 36. A DC magnetic field source 32, such as apermanent magnet, an example of which is a ferrite disc, can be mountedin such a manner as to provide a normal (perpendicular) magnetic fieldat the sensor 34. The concept of this arrangement is to provide anexternal magnetic field source to induce magnetization in any localferromagnetic body, so that the sensor 34 can detect that body, while,at the same time providing no in-the-plane-of-the-sensor active-axismagnetic field.

[0048] The use of a reference sensor helps to eliminate common modeerror signals. For instance, a nearby passenger conveyer, such as agurney, could contain magnetic components, but this spuriousmagnetization is not what is intended to detect, and, therefore, it ispreferable to eliminate this magnetic source.

[0049] An audio alert 37, such as a buzzer, and/or an alarm light 39 canbe employed to signal the presence of an unwanted ferromagnetic object.A ferromagnetic bobby pin is an example of such an unwantedferromagnetic object.

[0050] A non-ferromagnetic covering material, constructed, for instance,of a substance such as aluminum or nylon, or other suitable material,can surround the wand 30. This type of covering is not only protective;it also facilitates removal of any ferromagnetic objects which mightstick to the wand.

[0051] As shown in FIG. 7, the sensor's sensitivity axis is orthogonalto the axis of the magnetic field of the permanent magnet 32. Otherwisestated, the magnetic field of the permanent magnet 32 is normal to theplane of the sensor 34.

[0052] In FIG. 8, the magnetic field of the DC permanent magnet fieldsource 32 magnetizes the ferromagnetic object, which then has a magneticfield of its own, as shown in FIG. 9. This induced magnetization (“demagfield”) is detected by the sensor 34, triggering the alarm buzzer 37and/or light 39.

[0053] An alternative wand configuration, shown in FIG. 10, utilizes twopermanent magnets 32A, 32B, as the magnetic field between them is lessdivergent than with a single permanent magnet. With the use of twopermanent magnets 32A, 32B and less resultant divergence, there is lessneed for criticality about positioning the permanent magnet with respectto the sensor 34.

[0054] While the particular invention as herein shown and disclosed indetail is fully capable of obtaining the objects and providing theadvantages hereinbefore stated, it is to be understood that thisdisclosure is merely illustrative of the presently preferred embodimentsof the invention and that no limitations are intended other than asdescribed in the appended claims.

We claim:
 1. An apparatus for excluding ferromagnetic objects fromintroduction into a safe zone, comprising: an array of sensors adaptedto sense a magnetic field of a ferromagnetic object, said magnetic fieldbeing an induced magnetic field caused by magnetization of saidferromagnetic object by an external magnetic field; a processor adaptedto interpret signals from said sensor array sensing said magnetic fieldof said object; and a portal structure on which said sensor array ismounted, said portal structure being adapted to position the entirety ofsaid sensor array in proximity to a human subject, said portal structurebeing adapted to orient said sensor array to distinguish between saidexternal magnetic field and said magnetic field of said object.
 2. Theapparatus recited in claim 1, wherein said processor is further adaptedto interpret signals from said sensor array sensing said magnetic fieldto characterize said object.
 3. The apparatus recited in claim 1,wherein said processor is further adapted to interpret signals from saidsensor array sensing said magnetic field to locate said object.
 4. Theapparatus recited in claim 1, wherein said portal structure has at leastfirst and second vertical members, one of said vertical members beingarranged on each side of a passageway adapted for passage of a humansubject.
 5. The apparatus recited in claim 4, wherein: said sensor arraycomprises at least first and second sensor sub-arrays; said first sensorsub-array is arranged on said first vertical member on a first side ofsaid passageway; and said second sensor sub-array is arranged on saidsecond vertical member on a second side of said passageway.
 6. A methodfor excluding objects from a safe zone, said method comprising:providing an array of sensors adapted to detect an induced magneticfield of an object; positioning the entirety of said sensor array on aportal structure to scan a human subject; orienting said sensor arrayrelative to a source of an external magnetic field to distinguishbetween said external magnetic field and said induced magnetic field ofsaid ferromagnetic object; passing a human subject through said portalstructure to scan said human subject; and processing signals from saidsensor array to detect said object.
 7. The method recited in claim 6,further comprising interpreting said signals from said sensor array tocharacterize said object.
 8. The method recited in claim 6, furthercomprising interpreting said signals from said sensor array to locatesaid object.